Soil Solarization as an Alternative to Methyl Bromide In California Orchards

Solarization as a pre- or post-plant soil treatment to control soilborne pathogens and pests is a viable alternative to methyl bromide in orchard crops such as peaches, plums, nectarines, apricots, walnuts, pistachios, almonds, apples, and cherries. Currently, methyl bromide is used to control soilborne bacteria and diseases, weeds, nematodes, and fungi in these crops (DeVay 1995, Stapleton 1995, Pullman et al. 1984). As early as 1981, soil solarization was successfully used in California to control Verticillium wilt in pistachio tree groves (Ashworth and Gaona 1982). Since then, extensive soil solarization research has been conducted in orchards and the treatment is being appraised by many large orchard growers (McKenry 1996). In 1992, the top five California orchard uses of methyl bromide (e.g., almonds, nectarines, plums, peaches and walnuts) utilized over 2.5 million pounds of methyl bromide (State of California 1992).

Solarization is a hydrothermal process that can occur in moist soils covered with plastic tarps and exposed to direct sunlight in tropical climates or during warm summer months in more temperate regions. Solarization traps solar radiation, and thereby heat, in the soil and raises temperatures sufficiently to suppress or eliminate soil-borne pests and pathogens (Katan 1981, Katan and DeVay 1991). Solarization also causes complex changes in the biological, physical, and chemical properties of the soil that improve plant development, growth, quality, and yield for several years (Stapleton 1994, Katan and DeVay 1991, DeVay et al. 1990, Katan 1981). In areas with a suitable climate, solarization can be used alone, or in combination with lethal or sublethal fumigation or biological control, to provide an effective substitute to methyl bromide (Hartz et al. 1993).

In addition to disinfesting the soil while reducing or eliminating the need for fumigants, solarization leaves no toxic residues and can contribute to water conservation. Furthermore, solarization increases the levels of available mineral nutrients in soils by breaking down soluble organic matter and increasing bioavailablity. In doing so, solarization stimulates an increased growth response in many orchard trees and changes the soil microflora to favor biological pest control. Lastly, polyethylene films used in solarization can serve as mulch to reduce weeds when maintained as a row cover throughout the growing season (Stapleton 1994, Katan and DeVay 1991).

Soil Solarization in California Orchards

Unlike methyl bromide, soil solarization can be used effectively as both a pre- and post-plant treatment in many California (and other regional) orchards. Clear polyethylene films are typically used in pre-plant orchard treatments, while black polyethylene films (which achieve slightly lower temperatures depending on the thickness of the film) are most often used on newly planted or established orchards to gain the benefits of solarization while preventing heat damage to trees (DeVay 1996, Stapleton et al. 1993). Orchard trees have also been successfully established using clear polyethylene mulch as a pre-plant treatment in cooler areas of the San Joaquin and Sacramento Valleys (Stapleton and DeVay 1985, Stapleton et al. 1989).

Solarization causes physical, chemical, and biological changes in the soil by raising soil temperatures from 2-15 C above the temperatures of untreated soil. Soil solarization is successful because most plant pathogens and pests are mesophilic or unable to survive for long periods at temperatures above 37 C. Pathogens may be killed either directly by the heat or are weakened by sublethal heat to the extent that they are unable to damage crops (DeVay 1996). The heat sensitivity of these organisms is directly linked to an upper limit of fluidity in cell membranes, which lose their ability to function at high temperatures. Other methods of inactivation affected by solarization include sustained interference with enzyme systems, especially the respiratory process (DeVay et al. 1990).

In addition to providing pest and pathogen control, solarization conserves water and promotes growth in new orchards or replanted trees in temperate, as well as arid climates (Stapleton et al. 1993, 1991, and 1989, Duncan et al. 1992, Stapleton and Garza-Lopez 1988, Katan 1987, Stapleton and DeVay 1986). Experiments have confirmed that polyethylene films used for solarization conserve irrigation water under arid and drought conditions by preventing evaporation and trapping water. Furthermore, there is significant evidence that even in hot and arid climates, non-mature deciduous fruit and nut trees (e.g., almond, peach, apricot) may be established with no more than pre-plant irrigation and perhaps two or three carefully timed irrigations later in the season if necessary (Stapleton et al. 1993, Duncan et al. 1992, Stapleton et al. 1989, Stapleton and Garza-Lopez 1988). Solarization may also result in an increased growth response (as evidenced by increased trunk diameters) and yield in orchard trees, by increasing the availability of plant nutrients and the relative populations of beneficial organisms (i.e., rhizosphere bacteria (such as Bacillus spp. and Pseudomonas spp.), Trichoderma spp., actinomycetes, and mycorrhizal fungi) (Stapleton 1996, Katan and DeVay 1991, Stapleton et al. 1989, Stapleton and Garza-Lopez 1988, Pullman et al. 1984).

Solarization Techniques

The effectiveness of solarization and the heat dosages achieved during solarization depend on soil moisture and texture; air temperature (maxima, minima, and duration); season; length of day; intensity of sunlight; wind speed and duration; and the type, color, and thickness of the plastic (Katan and DeVay 1991, DeVay et al. 1990). Orchard trees create discontinuities in the field so that application of continuous plastic films must either be done manually or semimechanically using plastic-laying machinery. Plastic strips are cut and hand applied around tree bases and then (in the case of semimechanical applications) connected to sheets of machine-applied plastic between tree rows with heat-resistant glue or narrow bands of soil (Pullman et al 1984). While not as effective as the above, in some cases, wide strips of plastic are only placed between tree rows (strip mulching) or are applied by piercing films over young tree shoots in newly planted orchards (DeVay 1996, Katan and DeVay 1991).

In pre-plant orchard treatments, a layer of polyethylene film is applied to the soil prior to planting and is left in place for 4 to 6 weeks or more during the hot season. In post-plant treatment, however, polyethylene films are applied after planting and can remain in place for up to two years (McKenry 1996). Proper soil preparation is also essential to provide a smooth, even surface for the film and allow water to penetrate evenly and deeply into the soils (Stapleton 1996). To maintain proper soil moisture, orchards are irrigated 1 to 4 days prior to applying the plastic tarp. Alternatively, irrigation lines can be installed beneath the tarp and utilized as necessary (Katan 1981). While not currently field feasible, double layers of plastic can simulate solarization under glasshouse conditions, and will result in even greater temperature increases in soils (i.e., 3 to 10 C higher then that achieved under a single layer of plastic) (DeVay et al. 1990, DeVay 1996). Regardless of the technique used, the beneficial effects of solarization may persist for up to 2 years or more after the plastic is removed (Katan and DeVay 1991, DeVay et al. 1990).

Solarization Research In Orchards

A number of researchers have reported successful pre- and post-plant applications of soil solarization or other film mulching techniques for management of soilborne pests and pathogens in orchards. For example, solarization is known to control Verticillium wilt in pistachios (Ashworth and Gaona 1982) and olive trees (Tjamos et al. 1991, Katan and DeVay 1991), almonds and apricots (Stapleton, et al. 1993) and white root rot in apple trees (Freeman et al. 1990, Sztejnberg et al. 1987). Solarization is also effective against certain nematode species and non-specific replant diseases in other crops, such as almonds, peaches, and walnuts (Abu-Gharbieh et al. 1991, Stapleton et al. 1989, Jenson and Buszard 1988, Stapleton and DeVay 1984, 1983). Although solarization is an effective treatment method for a wide variety of orchard crops, crop response to solarization varies. For example, apricots are very responsive to soil solarization in that they are only susceptible to Verticillium wilt during the first 4 to 6 years of growth, therefore only one solarization treatment is required. Other orchard trees (i.e., certain cultivars of olive and pistachio); however, are susceptible to Verticillium wilt both in the first few years of growth and as mature trees and therefore must be treated repeatedly (Stapleton et al. 1993).

Although solarization can be a viable alternative to methyl bromide in orchards, there are limitations to it use. While solarization is just as effective as methyl bromide in the upper layers of the soil, the combined high heat levels and duration are often not adequate to penetrate into deeper soil levels (Stapleton 1995, DeVay 1995). This may impact overall yields when this is the only pest control tool utilized. Recent research; however, suggests that soil solarization, in combination with other alternatives to methyl bromide (e.g., TeloneŽ or VapamŽ) offers an "additive" effect that actually increases the efficacy of both chemical alternatives and solarization compared to their stand-alone uses. Although solarization is most effective in warm, arid climates; clear, thicker, and at even double layers of plastic (not currently feasible) can be used to achieve lethal levels of heat in more temperate regions (Katan and DeVay 1991). Although solarization has been successfully used in mature orchards, excessive shading by mature tree canopies may limit the effectiveness of this treatment under certain conditions (Stapleton et al. 1993, Stapleton et al. 1989).

Reducing Chemical Usage and Costs

Solarization can be a cost-effective technique and when the additional benefits of increased growth response, water conservation, and enhanced nutrient availability are considered, the economics are further improved (Stapleton 1994, Katan and DeVay 1991). Furthermore, solarization can be (and sometimes must be) combined with other chemical, physical, and biological methods (e.g., fertilizers, soil amendments, integrated pest management strategies, limited pesticide use, and biological control agents) for enhanced management of soilborne pests and pathogens (DeVay 1996, Katan and DeVay 1991).

The cost of solarization varies depending on the thickness of the plastic, areas of soil coverage (partial vs. complete coverage), irrigation methods, and the method of plastic application, connection, and removal (Pullman, 1984). For example, strip mulching can reduce solarization costs to two thirds the cost of full tarping (McKenry 1996). General cost estimates for solarization compared to methyl bromide fumigation are provided in Table 1 below. As mentioned above, chemical treatments can improve the control levels achieved with solarization. Therefore, representative chemical costs for TeloneŽ or VapamŽ have been included in the cost ranges presented in the table below. As shown, reduced chemical usage and cost savings can be achieved by using solarization for controlling soil-borne pests and pathogens. The direct costs of soil solarization can be one-half that of methyl bromide treatments (DeVay 1995; Stapleton 1995). Both this technique and the use of methyl bromide will require consideration of costs associated with the disposal or recycling of the plastic tarps.

Sources: DeVay 1996, McKenry 1996, PolyWest 1996, Asgrow 1995, Lukes Agrisales 1995, Helena Chemical 1995.

References

Abu-Gharbieh et al. 1991. Use of Black Plastic for Soil Solarization and Post-plant Mulching. In: DeVay J.E., Stapleton JJ. Elmore CE, eds. Soil Solarization. EAR. Rome. Plant Production and Protection Paper 109. pp. 229-242.

 

Asgrow 1995 (February). Personal communication. Asgrow. Price Quote for Telone C-17, Methyl Bromide, and Tillam.

 

Ashworth and Gaona 1982. Evaluation of Clear Polyethylene Mulch for Controlling Verticillium Wilt in Established Pistachio Nut Orchards. Phytopathology. Volume 72, pp. 243-246.

 

DeVay 1995 (January 18). Personal communication. J.E. DeVay. Professor (retired), Department of Plan Pathology University of California, Davis.

 

DeVay 1996 (September). Personal communication. J.E. DeVay. Professor (retired), Department of Plan Pathology University of California, Davis.

 

DeVay et al. 1990. Soil Solarization. J.E. DeVay, J.J. Stapleton, and C.L. Elmore. Food and Agricultural Organization, United Nations. FAO Report #109. Rome, Italy.

 

Duncan et al. 1992. Establishment of Orchards with Black Polyethylene Film Mulching: Effect on Nematode and Fungal Pathogens, Water Conservation, and Tree Growth. Journal of Nematology. Volume 24, pp. 681-687.

 

Freeman et al. 1990. Long-term Effect of Soil Solarization for the Control of Rosellinia necatrix in Apple. Crop Protection. Volume 9, pp. 312-316.

 

Helena Chemical 1995 (February). Personal communication. Helena Chemical. Price Quote for Telone C-17, Methyl Bromide, and Tillam.

 

Jensen and Buszard 1988. The Effects of Chemical Fumigants, Nitrogen, Plastic Mulch, and Metalazyl on the Establishment of Young Apple Trees in Apple Replant Disease Soil. Canadian Journal of Plant Science. Volume 68. pp. 255-260.

 

Katan and DeVay 1991. Soil Solarization. J. Katan and J.E. DeVay. CRC Press Inc. Boca Raton, Ann Arbor, Boston, London.

 

Katan 1981. "Solar heating (solarization) of soil for control of soilborne pests." J. Katan. Annual Review of Phytopathology. Volume 19, pp. 211-36.

 

Katan 1987. Soil Solarization. J. Katan. In: Innovative Approaches to Plant Disease Control. John Wiley & Sons, Inc. pp. 77-105.

 

Lakes Agrisales 1995 (February). Personal communication. Lykes Agrisales. Price Quote for Telone C-17, Methyl Bromide, and Tillam.

 

McKenry 1996 (September). Personal Communication. M.V. McKenry. University of California. Kearney Agricultural Center. Parlier, CA.

 

Pullman, G.S. et al. 1984. Soil Solarization, A Nonchemical Method for Controlling Diseases and Pests. G.S. Pullman, J.E. DeVay, C.L. Elmore, and W.H. Hart. Cooperative Extension Publication 21377, University of California.

 

PolyWest 1996 (June 15). Polyon Mulch and Low Tunnel Pricing. PolyWest. San Diego, CA.

 

Stapleton 1996 (September). Personal communication. James J. Stapleton. Statewide Integrated Pest Management Project, University of CA Kearney Agricultural Center.

 

Stapleton 1995 (January 20). Personal communication. James J. Stapleton. Statewide Integrated Pest Management Project, University of CA Kearney Agricultural Center.

 

Stapleton 1994. "Solarization as a framework for alternative soil disinfestation strategies in the interior valleys of California." J.J. Stapleton. In Proceedings of the 1994 Annual International Research Conference on Methyl Bromide Alternatives and Emissions Reductions. Kissimmee, FL.

 

Stapleton and DeVay 1986. Differentiation of Verticillium dahliae pathotypes and Cotton Tolerance to Wilt as Affected by Stem-puncture Inoculum Concentration (Abstract). J.J. Stapleton and J.E. DeVay. Phytopathology. Volume 76, p. 1107.

 

Stapleton and DeVay 1985. Soil Solarization as a Post-plant Treatment to Increase the Growth of Nursery Trees (Abstract). J.J. Stapleton and J.E. DeVay. Phytopathology. Volume 75, p. 1179.

 

Stapleton and DeVay 1984. Thermal Components of Soil Solarization As Related to Changes in Soil and Root Microflora and Increased Plant Growth Response. J.J. Stapleton and J.E. DeVay. Phytopathology. Volume 74, p.p. 255-259.

 

Stapleton and DeVay 1983. Response of Phytoparasitic and Free-living Nematodes to Soil Solarization and 1,3-dichloropropene in California. J.J. Stapleton and J.E. DeVay. Acta Horticulture. Volume 255, p.p. 161-168.

 

Stapleton and Garza-Lopez 1988. Mulching of Soils with Transparent (Solarization) and Black Polyethylene Films to Increase Growth of Annual and Perennial Crops in Southwest Mexico. Tropical Agriculture Trinidad. Volume 65, pp. 29-33.

 

Stapleton et al. 1993. Establishment of Apricot and Almond Trees Using Soil Mulching with Transparent (Solarization) and Black Polyethylene Film: Effects on Verticillium Wilt and Tree Health. J.J. Stapleton, E.J. Paplomatas, R.J. Wakeman, and J.E. DeVay. Plant Pathology. Volume 42, pp. 333-338.

 

Stapleton et al. 1991. Use of In-season Polyethylene Mulching for Establishment of Deciduous Fruit and Nut Trees in the San Joaquin Valley: Effects on Pathogen Numbers and Tree Survival. Proceedings of the National Agricultural Plastics Congress. Volume 23, pp. 260-265.

 

Stapleton et al. 1989. Use of Polymer Mulches in Integrated Pest Management Programs for Establishment of Perennial Fruit Crops. J.J. Stapleton, W.K. Asai, and J.E. DeVay. Acta Horticulture. Volume 255, pp. 161-168.

 

State of California 1992. Pesticide Use Report, Annual 1992.

 

Sztejnberg et al. 1987. Control of Rosellinia necatrix in Soil and Apple Orchard by Solarization and Trichoderma harzianum. Plant Disease. Volume 71, pp. 365-369.

 

Tjamos et al. 1991. Recovery of Olive Trees With Verticillium Wilt After Individual Application of Soil Solarization in Established Olive Orchards. Plant Disease. Volume 75, pp. 557-562.

Please note that this publication discusses specific proprietary products and pest control methods. Some of these alternatives are now commercially available, while others are in an advanced stage of development. In all cases, the information presented does not constitute a recommendation or an endorsement of these products or methods by the Environmental Protection Agency (EPA) or other involved parties. Neither should the absence of an item or pest control method necessarily be interpreted as EPA disapproval.